A perfluorocarbon copolymer containing functional groups is often used as a base material for a cation exchange membrane for electrolysis of brine, a diaphragm for fuel cells, etc.
As the perfluorocarbon copolymer containing functional groups, a perfluorocarbon copolymer containing carboxylic functional groups and a perfluorocarbon copolymer containing sulfonic functional groups are particularly useful.
For example, in the diaphragm electrolysis of brine by using the cation exchange membrane, a high-purity sodium hydroxide can be produced at high current efficiency and low cell voltage by arranging a layer having carboxylic acid type cation exchange groups on the side of the membrane facing the cathode compartment and a layer having sulfonic acid type cation exchange groups on the side of the membrane facing an anode compartment. The carboxylic acid type cation exchange groups are prepared by converting functional groups of the perfluorocarbon copolymer containing carboxylic functional groups to cation exchange groups. The sulfonic acid type cation exchange groups are prepared by converting functional groups of the perfluorocarbon copolymer containing sulfonic functional groups to cation exchange groups.
With respect to the techniques for producing the perfluorocarbon copolymer containing carboxylic functional groups or sulfonic functional groups, studies have been made on techniques using an aqueous medium and techniques using a non-aqueous medium.
Techniques using a non-aqueous medium are described in, for example, Japanese Patent Application Laid-Open No. 92026/1982, U.S. Pat. No. 3,528,954, U.S. Pat. No. 3,642,742, U.S. Pat. No. 4,138,426 and U.S. Pat. No. 4,267,364. Since the methods described therein use chlorofluorocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane as a reaction medium, these methods cause an ecological problem in that the medium destroys the ozone layer.
Techniques for producing the perfluorocarbon copolymer containing carboxylic functional groups in an aqueous medium are described in, for example, Japanese Patent Publication No. 53371/1982, U.S. Pat. No. 4,138,373, Japanese Patent Application Laid-Open No. 161810/1980 and U.S. Pat. No. 4,320,205. But when the perfluorocarbon copolymer containing carboxylic functional groups obtained by these techniques is used as a base material for a cation exchange membrane for electrolysis of brine, sufficiently high current efficiency cannot be obtained as yet.
Techniques for producing a perfluorocarbon copolymer containing sulfonic functional groups in an aqueous medium are described in, for example, Japanese Patent Application Laid-Open No. 160008/1980, U.S. Pat. No. 3,635,926, Japanese Patent Application Laid-Open No. 288617/1987, WO86/07075, WO94/03503 and Japanese Patent Application Laid-Open No. 184244/1994. According to the methods described in Japanese Patent Application Laid-Open No. 160008/1980 and U.S. Pat. No. 3,635,926, it is extremely difficult to stably produce the perfluorocarbon copolymer containing sulfonic functional groups, which has a low equivalent weight. According to the methods described in Japanese Patent Application Laid-Open No. 288617/1987, WO86/07075 and WO94/03503, the perfluorocarbon copolymer containing sulfonic functional groups, which has a lower equivalent weight, can be produced. However, the perfluorocarbon copolymer containing sulfonic functional groups obtained by the methods described therein has extremely poor melt moldability. For example, when an extrusion film is produced from the above copolymer by conventional melt extrusion, the film is often cut at the side edge and often has spots on the surface due to the uneven thickness and the surface is often rough so that a uniform and good film cannot be obtained. Particularly, it is difficult to produce a thin film which is industrially useful and indispensable. According to the method described in Japanese Patent Application Laid-Open No. 184244/1994, it is difficult to obtain a copolymer having a high molecular weight, which has a practically sufficient strength.
In order to solve the above problems, the present invention provides a method for producing a copolymer of a perfluorocarbon monomer containing a functional group and tetrafluoroethylene, which has extremely improved performance, in an aqueous medium, and also a copolymer containing functional groups obtained by the method, which has high performance. The perfluorocarbon copolymer containing functional groups has extremely improved melt moldability. Further it can be used as excellent base materials for a cation exchange membrane, with which electrolysis of brine can be conducted at high current efficiency and a low power consumption.